Implantable medication delivery devices are widely discussed in the technical and patent literature. They typically include a housing containing a medication reservoir which can be filled transcutaneously by a hypodermic needle penetrating a fill port septum. The medication reservoir is generally coupled via an internal flow path to a device outlet port for delivering medication to the patient. A typical delivery device further includes some type of mechanism, e.g., a propellant chamber, for moving the medication from the reservoir through the internal flow path to the device outlet port for delivery to the patient.
The literature recognizes that overfilling the medication reservoir can result in unintended medication delivery from the device outlet port to the patient. Accordingly, various techniques have been proposed for avoiding problems associated with overfilling. For example, U.S. Pat. No. 5,158,547 describes an overfill protection mechanism comprising a valve associated with the fill port which automatically responds to a “reservoir full” condition to dose the fill port to prevent overfilling.